How do you switch from knit to purl on same row?

[Rebecca419]

I apologize if this has already been answered somewhere.

I made a simple scarf now I am trying to move on up with a dishcloth. It calls for knitting and then purling, but I don't know how to switch from one to the other on the same row without it getting messed up (since it needs to be in the back for knitting and in the front for purling)

Here is what I am trying to do:

http://www.blackrayne.com/pages/skully.html

Thank you!!